Barbara Lafuente

Implications from chemical, structural and mineralogical studies of magnetic microspherules from around the lower younger dryas boundary (New Mexico, Usa)

Abstract Hollow magnetic microspherules from along the lower Younger Dryas boundary (c. 12.9 ka bp) in New Mexico (USA) were studied using scanning electron microscopy, electron probe microanalysis, X‐ray diffraction, and laser‐ablation inductively coupled‐plasma mass spectrometry methods. The shell of the microspherules (10–15% of the spherules diameter) displays dendritic surface textures, which are likely due to quenching during rapid cooling of molten material. Structurally, multiple single‐magnetite crystals attached together form the bulk of the microspherules. Iron dominates the microspherules’ composition (∼90% FeOtot), Mn is the second most abundant element (up to 0.4% MnO), Al is detected in low concentrations (<0.30% of Al2O3). Among the trace elements, the rare earth elements display slightly fractionated patterns with concentrations of 0.1–1.0× CI chondrite. The microspherules contain elevated concentrations of Ni relative to detrital magnetite (up to 435 ppm) and very low concentrations of Ti (down to 5 ppm). Chemical, structural and mineralogical features of the microspherules do not contradict the existing models of the formation during ablation while a meteoroid goes through the Earths atmosphere. Elevated concentrations of the magnetic microspherules in sediments can be a stratigraphic marker for the lower Younger Dryas boundary in North America.

Crystal structure of tetra­wickmanite, Mn2+Sn4+(OH)6

The crystal structure of tetrawickmanite, a tetragonal hydroxide-perovskite mineral, has been determined for the first time by means of single-crystal X-ray diffraction. It is characterized by alternating corner-linked [Mn2+(OH)6] and [Sn4+(OH)6] octahedra whose sense of rotation varies along c, in contrast to its dimorph, the cubic wickmanite.

Refractive indices of minerals and synthetic compounds

Abstract This is a comprehensive compilation of refractive indices of 1933 minerals and 1019 synthetic compounds including exact chemical compositions and references taken from 30 compilations and many mineral and synthetic oxide descriptions. It represents a subset of about 4000 entries used by Shannon and Fischer (2016) to determine the polarizabilities of 270 cations and anions after removing 425 minerals and compounds containing the lone-pair ions (Tl+, Sn2+, Pb2+, As3+, Sb3+, Bi3+, S4+, Se4+, Te4+, Cl5+, Br5+, I5+) and uranyl ions, U6+. The table lists the empirical composition of the mineral or synthetic compound, the ideal composition of the mineral, the mineral name or synthetic compound, the Dana classes and subclasses extended to include beryllates, aluminates, gallates, germanates, niobates, tantalates, molybdates, tungstates, etc., descriptive notes, e.g., structure polytypes and other information that helps define a particular mineral sample, and the locality of a mineral when known. Finally, we list nx, ny, nz, (all determined at 589.3 nm), , deviation of observed and calculated mean refractive indices, molar volume Vm, corresponding to the volume of one formula unit, anion molar volume Van, calculated from Vm divided by the number of anions (O2−, F−, Cl−, OH−) and H2O in the formula unit, the total polarizability , and finally the reference to the refractive indices for all 2946 entries. The total polarizability of a mineral, , is a useful property that reflects its composition, crystal structure, and chemistry and was calculated using the Anderson-Eggleton relationship αAE=(nD2−1)Vm4π+(4π3−c)(nD2−1)

Crystal structure of tetrawickmanite, Mn 2+ Sn 4+ (OH) 6

\begin{array}{} \displaystyle \alpha_{\text{AE}}=\frac{\Big(n_{\rm D}^2-1\Big)V_{\rm m}}{4\pi+\Bigg(\displaystyle \frac{4\pi}{3}-c\Bigg)\Big(n_{\rm D}^2-1\Big)} \end{array}

Calcioferrite with composition (Ca3.94Sr0.06)Mg1.01(Fe2.93Al1.07)(PO4)6(OH)4·12H2O.

where c = 2.26 is the electron overlap factor. The empirical polarizabilities and therefore, the combination of refractive indices, compositions, and molar volumes of the minerals and synthetic oxides in the table were verified by a comparison of observed and calculated total polarizabilities, derived from individual polarizabilities of cations and anions. The deviation between observed and calculated refractive indices is <2% in most instances.

The power of databases: The RRUFF project

The crystal structure of brackebuschite, ideally Pb2Mn3+(VO4)2(OH), was redetermined based on single-crystal X-ray diffraction data of a natural sample from the type locality Sierra de Cordoba, Argentina. Improving on previous results, anisotropic displacement parameters for all non-H atoms were refined and the H atom located, obtaining a significant lowering of the reliability factors.

Sedimentary differentiation of aeolian grains at the White Sands National Monument, New Mexico, USA

The crystal structure of tetrawickmanite, a tetragonal hydroxide-perovskite mineral, has been determined for the first time by means of single-crystal X-ray diffraction. It is characterized by alternating corner-linked [Mn2+(OH)6] and [Sn4+(OH)6] octahedra whose sense of rotation varies along c, in contrast to its dimorph, the cubic wickmanite.

Lipid-mediated protein functionalization of electrospun polycaprolactone fibers

Abstract Hemihedrite from the Florence Lead-Silver mine in Pinal County, Arizona, USAwas first described and assigned the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2F2, based upon a variety of chemical and crystalstructure analyses. The primary methods used to determine the fluorine content for hemihedrite were colorimetry, which resulted in values of F that were too high and inconsistent with the structural data, and infrared (IR) spectroscopic analysis that failed to detect OH or H2O. Our reinvestigation using electron microprobe analysis of the type material, and additional samples from the type locality, the Rat Tail claim, Arizona, and Nevada, reveals the absence of fluorine, while the presence of OH is confirmed by Raman spectroscopy. These findings suggest that the colorimetric determination of fluorine in the original description of hemihedrite probably misidentified F due to the interferences from PO4 and SO4, both found in our chemical analyses. As a consequence of these results, the study presented here proposes a redefinition of the chemical composition of hemihedrite to the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2. Hemihedrite is isotypic with iranite with substitution of Zn for Cu, and raygrantite with substitution of Cr for S. Structural data from a sample fromthe Rat Tail claim, Arizona, indicate that hemihedrite is triclinic in space group P1, a = 9.4891(7), b = 11.4242(8), c = 10.8155(7) Å, α = 120.368(2)°, β = 92.017(3)°, γ = 55.857(2)°, V = 784.88(9) Å3, Z = 1, consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R1 = 0.022 for 5705 unique observed reflections, and the ideal chemical formula Pb10Zn(CrO4)6(SiO4)2(OH)2 was assumed during the refinement. Electron microprobe analyses of this sample yielded the empirical chemical formula Pb10.05(Zn0.91Mg0.02)Σ = 0.93(Cr5.98S0.01P0.01)Σ = 6.00 Si1.97O34H2.16 based on 34 O atoms and six (Cr + S + P) per unit cell.